Electricity is an essential part of modern life. Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to electrical substations located near demand centres. Transmission lines mostly use high-voltage three-phase alternating current (AC). Electricity is transmitted at high voltages (110 kV or above) to reduce the energy lost in long-distance transmission. Power is usually transmitted through overhead power lines. Underground power transmission has a significantly higher cost and greater operational limitations but is sometimes used in urban areas or sensitive locations. Most recently, submarine power cables provide the possibility to supply power to small islands or offshore production platforms without their own electricity production. On the other hand, submarine power cables also provide the possibility to bring ashore electricity that was produced offshore (wind, wave, sea currents . . . ) to the mainland.
These power cables are normally steel wire armoured cables. A typical construction of steel wire armoured cable 10 is shown in FIG. 1. Conductor 12 is normally made of plain stranded copper. Insulation 14, such as made of cross-linked polyethylene (XLPE), has good water resistance and excellent insulating properties. Insulation 14 in cables ensures that conductors and other metal substances do not come into contact with each other. Bedding 16, such as made of polyvinyl chloride (PVC), is used to provide a protective boundary between inner and outer layers of the cable. Armour 18, such as made of steel wires, provides mechanical protection, especially provides protection against external impact. In addition, armouring wires 18 can relieve the tension during installation, and thus prevent copper conductors from elongating. Possible sheath 19, such as made of black PVC, holds all components of the cable together and provides additional protection from external stresses.
In use, submarine cables are generally installed under water, typically buried under the bottom ground or sea bed, but portions thereof may be laid in different environment; this is, for example, the case of shore ends of submarine links, intermediate islands crossing, contiguous land portions, edge of canals, transition from deep sea to harbor and similar situations. Associated with these environments, it is often a worse thermal characteristics and/or higher temperature with respect to the situation in the offshore or ashore main route.
The current rating, i.e. the amount of current that the cable can safely carry continuously or in accordance to a given load is an important parameter for an electric power cable. If the current rating is exceeded for a long time, the increase in temperature caused by the generated heat may damage the conductor insulation and cause permanent deterioration of electrical or mechanical properties of the cable. Therefore, the configuration of a power cable, e.g. the dimension of the core, is determined by the current rating. The current rating of a cable is dependent on the cable core size, the operational system parameters of the electric power distribution circuit, the type of insulation and materials used for all cable components and the installation condition and thermal characteristics of the surrounding environment.
In an AC power cable, the magnetic field generated by the current flowing in the conductors induces magnetic losses in ferromagnetic materials, or in a material having high magnetic permeability, such as in carbon steels used as armouring wires. The magnetic loss causes (or is transferred into) heat in the materials. Such an induced heat, added to the heat produced by the conductors due to the current transport, can limit the overall current carrying capacity of the power cable, especially when the power cable is deployed in environment with low or insufficient heat dissipation capability.
Solutions have been investigated to avoid a reduction in the electrical power transport capability of an electric cable due to heat generated by losses in the cable armouring.
One proposal is by increasing the size of the cable, particular of those cable sections which lay in the conditions of insufficient heat dissipation. However, such a solution is not desirable since it implies heavier and more expensive cables. A disadvantage of having a cable made of distinct sections of different size is that the cable continuity is impaired which is detrimental for the cable mechanical resistance, and it requires special transition joints between cable sections and requires careful handling during laying operation. In addition, these transition joints of the electric transmission cable may also generate additional electrical losses.
U.S. patent application publication No. 20120024565 discloses another solution to solve this problem. It discloses an electric power transmission cable comprising one first section provided with cable armour made of a first metallic material, and one second section provided with cable armour elements made of a second metallic material. The second metallic material is substantially free from ferromagnetism. The first and second sections are longitudinally contiguous with each other and an anticorrosion protection is provided in correspondence with a contact point between the armour elements in the first section and the armour elements in the second section. The anticorrosion protection comprises zinc rods or strips inserted in between the armour elements in the first section and the armour elements in the second section. According to this proposed solution, additional zinc rods or strips should be attached in the additional sleeve or belt joining the first section with the second section and thus the production of the power cable becomes complex and expensive.